It is sometimes required to superimpose an optical image derived electronically on to an external scene viewed through an eyepiece by an observer so that the observer can see both the electronic image as well as the external scene together. This requirement has been addressed in the prior art so as to permit, for example, a pilot to see flight data superimposed on an external scene.
U.S. Pat. No. 5,162,828 (Furness et al.) discloses a display system for conventional spectacles having a transparency that defines a field of view and a frame for supporting the transparency on a user's head. The display system includes a display mounted on the frame of the spectacles and optics for collimating light to project an image of the displayed information at a distance from the user in the periphery of the field of view defined by the transparency. According to one embodiment, the optics includes a single mirror which receives the information directly from the display and projects an enlarged image at an apparent optical distance from the user that is greater then the actual optical path length. Alternatively, a planar mirror and collimating lens may be employed.
U.S. Pat. No. 4,516,157 (Campbell) discloses a portable video recording device arranged to be worn by the user like a pair of spectacles. In the recording mode, the scene in front of the user is recorded on a miniature video camera mounted on the spectacle frame, allowing the user to view the scene normally, and also examine a viewfinder showing the portion of the scene available to the video camera. This enables the user to perform hands free photography.
A principal application of the device disclosed by Campbell is for detectives and the like to record situations unbeknown to a subject being photographed. To this end, miniature CCD elements and minimal optics are mounted behind a partially silvered spectacle frame so to be undetectable from the front of the spectacle frame. In order that the user may know accurately the field of view seen by the camera, a rectangular frame is provided on an inside surface of one of the spectacle lenses and the camera optics are pre-calibrated so as to focus on an external scene which is also seen by the user within the frame.
Such systems are commonly mounted on, or in conjunction with, a pilot's head up display and typically employ a miniature CCD TV camera mounted either on the head up display or to a visor thereof. For example, U.S. Pat. No. 5,341,242 to Gilboa et al. discloses an optical projector which projects an image on to the semi-transparent portion for reflection from it. The semi-transparent portion has a relatively higher reflectivity in a central area and a relatively lower reflectivity in an annular area outside the central area. In accordance with one embodiment, the head mounted display includes a projector having a display source where an image is formed. An optical system for focusing the image at separate sagittal and tangential foci is provided between the image-forming surface and the semi-transparent portion. Such an arrangement may be employed in a visor attached to a pilot's helmet, such that the visor has a partially reflecting semi-transparent portion in view of the pilot.
U.S. Pat. No. 4,398,799 (Swift) discloses a pilot's head-up display wherein an image of selected display information is superimposed by means of a combiner on an external scene and reflected towards the pilot's eyes by means of a mirror disposed on his helmet in such a manner so as to not to interfere with his vision.
U.S. Pat. No. 5,113,177 (Cohen) discloses a display system for enabling a pilot in an aircraft to view simultaneously an image of selected display information and a scene external to the aircraft during low visibility conditions, such as a night-time. An image intensifier tube is mounted on the side of a pilot's helmet so that the image sensed coincides with the pilot's field of view. The image intensifier tube intensifies light from the external scene and output voltages representative of the scene to a converter whose output is itself converted to a suitable video signal. A combining circuit combines the video signal with video signals from other sources, such as instrumentation panel displays. The combined electrical signals are fed to a display driver so as to allow superimposition of the selected display information on the external scene viewed by the pilot.
In the above systems described by Gilboa et al, Swift and Cohen and in all similar systems, the projected image is collimated so that the physical location of the projector relative to the visor is not critical.
U.S. Pat. No. 4,818,065 (Ziph) discloses an optical device particularly useful as a night vision goggles including an objective lens which is located remote from the viewer's eye for viewing a scene, and a cathode ray tube and prism assembly for injecting additional information into the viewed scene. Since the objective lens is adjusted to view the scene from infinity or near infinity, the incoming light must be near collimated. U.S. Pat. No. 4,629,295 (Vogl) discloses a night vision instrument which may be located beside an ordinary telescope eyepiece or TV camera so as to enable the telescope eyepiece or TV camera to be used during night time. A light gathering objective lens is provided having therein a concave mirror for imaging light rays from a scene towards an entrance surface of the image intensifier tube and an opposite exit surface for directing an intensified image via a first reflecting surface through a collimator and a second reflecting surface towards the telescope eyepiece. The second reflecting surface may, if desired, be a beam splitter so as to direct a part of the intensified image towards the eyepiece whilst allowing transmission of a portion of the intensified beam through the beam splitter towards the television camera mounted outside the objective.
It is thus to be noticed that, in the device proposed by Vogl, an external television camera may be effectively coupled, via a beam splitter, to an objective lens. Further, since the intensified image beam is directed through a collimator towards the beam splitter, the beam splitter sees a collimated beam suitable for viewing by a conventional infinite conjugate. Thus, in particular, it is to be noted that the arrangement disclosed by Vogl allows for the beam splitter to intercept a collimated beam and allows the beam splitter to be disposed in conjunction with an objective lens so as to couple the intensified image beam produced thereby to an eyepiece. There is also disclosed the possibility of allowing the beam splitter to direct a daylight image of a scene directly to the telescope eyepiece at the same time as the intensified night vision is directed thereto via the objective lens. However, even in this case, the beam splitter directs a conventional collimated beam to the eyepiece. Moreover, since the beam splitter is disposed outside of the eyepiece and, in fact, outside of the objective lens also, there are no limiting constraints on the physical separation of the beam splitter from either of these elements apart, of course, from the obvious requirement to avoid rendering the device unduly bulky.
FIGS. 1a and 1b show schematically prior art approaches for superimposing a display image 1 on an image of an external scene 2 so as to be viewed simultaneously by an observer 3. Common to both approaches is a light gathering objective lens 4, an image intensifier tube 5 or a similar device and an eyepiece 6. A beam splitter 7 combines light from the display image 1 with the external scene 2 so as to direct a combined image to the observer 3. In both arrangements, the image intensifier tube 5 is pre-calibrated so that its output is fairly constant.
The arrangement shown in FIG. 1a is similar to that taught by Ziph and described above. The beam splitter 7 is disposed before the light gathering objective lens 4 so that the display image 1 as well as the external scene 2 are both directed to the image intensifier tube 5 and amplified thereby. This arrangement suffers from the drawback that if the contrast between the brightness of the two images is high, then the low brightness component will be swamped by the bright component so as to be barely amplified by the image intensifier tube 5 in order to maintain the required constant brightness at its output. Consequently, the low brightness component will be barely visible to the observer 3. In order to compensate for this, the display image 1 must be monitored so as to render its brightness of a similar order to that of the external scene 2. Furthermore, since the external scene 2 effectively emanates from infinity, the beam splitter 7 sees a collimated beam suitable for viewing by a conventional infinite conjugate.
The arrangement shown in FIG. 1b is similar to that taught by Vogl and described above, in that the beam splitter 7 is placed after the eyepiece 6 so that only the external scene 2 is intensified prior to combining with the display image 1. This arrangement overcomes the problem of high brightness contrast associated with the arrangement shown in FIG. 1a. However, it requires that the beam splitter 7 be disposed between the eyepiece 6 and the eyes of the observer 3, thereby decreasing the eye relief and reducing comfort to the observer 3. Here also, as noted above with regard to Vogl, the beam splitter 7 intercepts the collimated beam produced by the objective lens 4 and amplified by the image intensifier tube 5 so as to superimpose the display image on the intensified image produced thereby.
Common to all such systems is that they employ an infinite conjugate which forms a collimated virtual image of a distant object. Since the image beam is collimated, the beam splitter can be disposed anywhere in the optical path of the image beam in order to achieve the required effect.
However, optical systems are also known which form a real image on a screen close to the eyes of the observer, and in such systems the image beam does not emanate from infinity and so is not collimated. Furthermore, such systems are usually compact, and it is therefore inherently more difficult to intercept the image beam so as to superimpose thereon a display image which is injected through the beam splitter.
One system which produces a real image of a distant object is a pair of Night Vision Goggles. Such goggles comprise, for each eye of the observer, an objective lens for imaging a distant object so as to form an image beam and an image intensifier tube which intercepts the image beam, converts the light to electrons and then amplifies the number of electrons. A phosphor captures the electrons so as to generate an amplified image which is viewed by an observer through a suitable eyepiece mounted proximate the image intensifier tube so that the phosphor is in the focal plane of the eyepiece.
Night vision goggles are compact and it is difficult both physically and optically to inject the image owing to the compactness of the eyepiece, particularly if optical distortions are to be avoided or at least compensated for.
Yet another consideration is that because space within the eyepiece is invariably at a premium, a beam splitter, for example, disposed therein in order to combine the eyepiece beam with the display image must be asymmetrical in order to maximize transmission of light through the eyepiece whilst still allowing the display beam to be injected without diminishing the field of view. Eyepieces of the kind generally used in night vision systems are usually provided with a wide angle so as to provide an increased field of view. Wide angle coverage is usually accomplished by providing several optical elements any pair of which are necessarily close to each other. It is into the limited space between these optical elements that any beam splitter must be disposed in order to allow superimposition of the display beam on to eyepiece image. If, for example, the aperture of the lens is in the order of 30 mm whilst the space between the optical elements is in the order of 10 mm, then a symmetrical beam splitter having equal dimensions in the order of 10 mm disposed symmetrically in the light path of the eyepiece, would intercept incoming light only within a 5 mm radius of the optical axis and there would be an annular portion having a width of 10 mm surrounding the beam splitter where light would pass directly through the eyepiece without passing through the beam splitter. This is unacceptable because it would introduce severe optical distortions in the eyepiece which would be most difficult to correct. Equally, it is unacceptable to block out the surrounding annular portion since this would drastically reduce both the field of view and the quantity of light passing through the eyepiece and is therefore obviously unacceptable during night time vision when, in any case, the quantity of light is limited. These considerations dictate the use of an asymmetric beam splitter which is, of course, distinguished from prior art designs of the kind discussed where, to the extent that beam splitters are used at all, they are invariably symmetrical.